Not Receiving OTP from End User Messaging

It's not uncommon to face challenges when trying to verify a new destination number in AWS End User Messaging, especially when you're in the SMS sandbox environment. There are a few potential reasons why you might not be receiving the One-Time Password (OTP):

1. Sandbox Restrictions: When your account is in the SMS sandbox, there are limitations on sending messages. You can only send to verified numbers, which creates a catch-22 situation when trying to verify a new number.

2. Origination Identity: For US numbers, you need to have an origination identity set up before you can send verification messages. This could be a Toll-Free Number (TFN), a 10-digit long code (10DLC), or a short code. If you haven't set this up, it could explain why you're not receiving the OTP.

3. Phone Number Format: Ensure you're entering the phone number in the correct format, including the country code (e.g., +1 for the United States).

4. Spam Filters: Check your phone's spam or blocked messages folder. Sometimes, verification messages can end up there.

5. VoIP Numbers: If you're using a VoIP number, there might be compatibility issues. Some verification systems treat VoIP numbers differently from standard mobile numbers.

To resolve this issue, you could try the following steps:

1. Verify that you have set up an origination identity (TFN, 10DLC, or short code) for your account.
2. Double-check the phone number format you're entering.
3. Try using a different phone number if possible.
4. If the issue persists, it may be worth contacting AWS Support for further assistance, as there could be an account-specific issue or a temporary service problem.

Remember, the process for fully validating a US number in the sandbox environment involves several steps, including registering a brand, registering a campaign, and requesting an originating ID. These steps can take a few days to complete.
  

Sources
facing challenges getting a US phone number verified for AWS End User Messaging in sandbox | AWS re:Post
Verified number doesn't send password | AWS re:Post
Can't receive verification message of Destination phone in SMS sandbox | AWS re:Post